Scientists have developed a technique for retrieving fingerprints from bullet casings and bomb fragments after they have been fired or detonated. The new method, which relies on subtle corrosion of metal surfaces is already being applied for the first time anywhere in the world by two British police forces.

The patterns of corrosion remain even after the surface has been cleaned, heated to 600C or even painted over. This means that traces of fingerprints stay on the metal long after the residue from a person's finger has gone.

"All other conventional techniques that the police anywhere in the world would use require some kind of either physical or chemical interaction with the fingerprint residue. So for example if you are using powder the powder sticks to the tackiness in the sweat," said Dr John Bond of the University of Leicester and the Scientific Support Unit of Northamptonshire Police. Instead, the technique he has developed relies on permanent physical changes to the metal.

The chemical basis of the change is not yet clear, but Bond believes it is corrosion by chloride ions from the salt in sweat. These produce lines of corrosion along the ridges of the fingerprint residue. When the metal is heated, for example in a bomb blast or when a gun is fired, the chemical reaction actually speeds up and makes the corrosion more pronounced.

To visualise the patterns, Bond's technique involves passing 2500 volts through the metal and sprinkling a fine conducting powder on it. While the electric field is applied, the powder sticks to the corroded areas.

"If the corrosion's there, it will stay there unless you abrasively clean the metal. The only way to get rid of it is to actually take the surface layer of the metal off," he said. "Some metals corrode very well and some not at all. One metal that works very well is brass and gun cartridge cases happen to be made of brass."

He has published details of the technique in the latest issue of the Journal of Forensic Sciences. In his paper he demonstrates that it is possible to recover fingerprints from a bullet casing ejected when a pistol is fired. "As you are pushing the magazine in you are actually putting a thumb print on the bullet," said Bond. "That's the person you want. That's the guy who loaded the gun."

The Metropolitan Police has asked Bond to look into evidence kept from a 10 year old murder case. He is also using the technique on evidence from a recent murder that is being investigated by Lincolnshire police.